The present invention relates to a power amplification apparatus, particularly to an apparatus for amplifying audio signals.
The typical tendency to the reduction of the power consumption and of the weights and encumbrances of the heat dissipators has induced to require power audio amplifiers with a higher efficiency than the class-AB amplifiers from the apparatus constructors.
Class-D audio amplifiers have been proposed for satisfying such request, which comprise a DC-AC converter producing a pulse width modulated output signal (PWM); such signal in turn drives power switches driving a load provided with a passive filter for restoring the amplified audio signal.
A single output amplifier with analog input and PWM output (class-D amplifier) is described in the article xe2x80x9cAnalysis of a quality class-D amplifierxe2x80x9d, F. A. Himmelstoss, et al., I.E.E.E. Transactions on Consumer Electronics, Vol. 42, No. 3, August 1996.
The more and more wide interest toward the digital processing of the signals has induced to provide power amplifiers with analog or digital input.
A particular class of such power amplifier is that of the audio switching amplifiers wherein the output of the power PWM block amplifier 1 is separated from a load 2 (generally a loudspeaker) by means of a LC low-pass filter 3, as shown in FIG. 1. When the amplifier is turned on by means of the signal En, in absence of input audio signal, the voltage signal detected before the LC filter 3 is a square wave with a duty-cycle of 50%, that is the square wave signal of the modulator PWM which has been amplified. The voltage signal Vout which is in output from the LC filter 3, which in running state has a waveform schematically formed by a triangular wave with a void middle value, in the period Ti following the turning on has a non-void middle value which causes a noise audible to the loudspeaker, as shown in FIG. 2. An equal result is achieved during the turning off of the amplifier, in the period Tf. The wideness of such noise also is not constant but it depends on the instant of application of the turning on signal.
A solution actually used for eliminating such noises is to interpose between the LC low-pass filter 3 and the loudspeaker a relay 4 suitably driven by means of a signal Enrelais which is activated after a certain time period Ton from the turning on and is deactivated at a time Toff before the turning off of the device, so that the loudspeaker is excluded in the periods interested from the transient, as shown in FIGS. 3 and 4.
In view of the state of the art described, it is an object of the present invention to provide a power amplification apparatus which reduces the noises due to the turning on and turning off transients of the amplifier in a economic way.
According to the present invention, such object is attained by a power amplification apparatus receiving in input an enable signal and an input square wave signal, characterized by comprising a device which receives said input square wave signal and said enable signal and which produces a new enable signal for said apparatus which is synchronized with a rise or down front of the input square wave signal, so that an output square wave signal of said apparatus, which is normally shifted of a certain period fraction with respect to the square wave signal in input to the apparatus, has the first and the last pulses which have a duration equal to a period fraction of said output square wave signal.